http://wiki.musicbrainz.org/Work#Work-to-Work_relationships , down at "Derivative work relationship" states that "A work can be expressed as being derived from one or more other works. Examples: instrumental work with lyrics added later, translation of a work into a different language, remix, mashup."

However, the last two aren't possible at the moment, given the work ARs in place. There is no artist-work AR for remixer, nor for the person who makes the mashup. There also is no work-work AR to indicate one work as being a mashup or remix of another.